Capacitance voltage conversion circuit, input apparatus using the same, electronic instrument, and capacitance voltage conversion method

ABSTRACT

A capacitance voltage conversion circuit, which converts respective capacitances of a plurality of sensor capacitors into voltages, includes: a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each capacitance current conversion circuit configured to generate a detection current corresponding to a capacitance of a corresponding sensor capacitor; a current average circuit configured to average a plurality of detection currents, which are respectively generated by the capacitance current conversion circuits, to generate an average current; and a plurality of current voltage conversion circuits disposed in respective correspondence with the sensor capacitors, each current voltage conversion circuit configured to convert a difference current between a corresponding detection current and the average current into a voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patent application Ser. No. 13/367,904, filed on Feb. 7, 2012, the entire contents of which are incorporated herein by reference and priority to which is hereby claimed. Application Ser. No. 13/367,904 claims the benefit of priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) from Japanese Patent Applications Nos. 2011-025019, filed on Feb. 8, 2011, and 2011-182223, filed on Aug. 24, 2011, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a capacitance voltage conversion circuit which is used to measure capacitance.

BACKGROUND

Recent electronic instrument such as computers, portable terminals, and Personal Digital Assistants (PDAs) generally include an input device for manipulating the electronic instrument by touching the input device with a finger or approaching a finger to the input device. As such an input device, joysticks, touch pads, etc. have been known.

Further, as a sensor, capacitance sensors using capacitance have been known. The capacitance sensors may include a sensor electrode. When a user approaches or touches the sensor electrode, capacitance generated by the sensor electrode is changed. By converting the capacitance change into an electric signal with a capacitance voltage conversion circuit, whether a user touches the sensor electrode or not is determined.

Touch panels may be configured with a plurality of sensor electrodes. Touch panels having an X-Y matrix type include row sensor electrodes that are disposed in each row of the matrix, and column sensor electrodes that are disposed in each column of the matrix. By detecting the capacitance change of each of a plurality of sensor electrodes, coordinates touched by a user are specified.

SUMMARY

A conventional capacitance detection circuit generally detects the capacitances of a plurality of sensor electrodes in a time division method. For example, an X-Y matrix type touch panel sequentially detects the capacitances of the column sensor electrodes, and sequentially detects the capacitances of the row sensor electrodes. In such a method, since the timings of capacitance detection for the respective sensor electrodes are different, the sensor electrodes may be respectively affected by different noises.

The present disclosure provides some embodiments of a capacitance voltage conversion circuit that can simultaneously detect the capacitances of a plurality of sensor electrodes.

According to one embodiment of the present disclosure, provided is a capacitance voltage conversion circuit, which converts a capacitance of each of a plurality of sensor capacitors into a voltage, including: a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each of the capacitance current conversion circuits configured to generate a detection current corresponding to the capacitance of the corresponding sensor capacitor; a current average circuit configured to average the detection currents, which are generated by the capacitance current conversion circuits, to generate an average current; and a plurality of current voltage conversion circuits disposed in respective correspondence with the sensor capacitors, each of the current voltage conversion circuits configured to convert a difference current between the corresponding detection current and the average current into a voltage.

In the present embodiment, each of the detection currents may correspond to a capacitance of a corresponding sensor capacitor, and the average current may correspond to an average capacitance of the capacitances of the sensor capacitors. Therefore, a voltage that is generated by the current voltage conversion circuit of each channel may indicate a difference between the capacitance of the sensor capacitor of each channel and an average capacitance of the capacitances of the sensor capacitors of all channels. According to such a configuration, the capacitances of the respective sensor capacitors can be detected simultaneously.

According to another embodiment of the present disclosure, provided is a capacitance voltage conversion circuit, which converts a capacitance of each of a plurality of sensor capacitors into a voltage, including: a plurality of reset switches disposed in respective correspondence with the sensor capacitors, each of the reset switches configured to initialize an electric potential of a corresponding sensor capacitor; a plurality of integral capacitors disposed in respective correspondence with the sensor capacitors, an electric potential of one end of each integral capacitor being fixed; an initialization circuit configured to initialize respective voltages of the integral capacitors; a plurality of current mirror circuits disposed in respective correspondence with the sensor capacitors, each of the current mirror circuits configured to supply a current, which flows in a second transistor of an output side of each of the current mirror circuits, to a corresponding integral capacitor in a first direction, a first transistor of an input side of each of the current mirror circuits being connected to a corresponding sensor capacitor; and a current average circuit configured to generate an average current of a plurality of detection currents which respectively flow in the sensor capacitors, and supply the average current to the integral capacitors in a second direction.

According to the present embodiment, in each channel, an integral capacitor may be charged (or discharged) by a detection current of a corresponding channel, and the capacitances of the respective sensor capacitors can be simultaneously detected by discharging (or charging) a corresponding capacitor by the average current.

According to another embodiment of the present disclosure, provided is a capacitance voltage conversion circuit, which converts a capacitance of each of a plurality of sensor capacitors into a voltage, including: a plurality of integral capacitors disposed in respective correspondence with the sensor capacitors, an electric potential of one end of each of the integral capacitors being fixed; a plurality of reset switches disposed in respective correspondence with the sensor capacitors, each of the reset switches being connected to a corresponding sensor capacitor in parallel; a plurality of sensing switches disposed in respective correspondence with the sensor capacitors, one end of each of the sensing switches being connected to a corresponding sensor capacitor; a plurality of first transistors, being MOSFETs, disposed in respective correspondence with the sensor capacitors, each of the first transistors being disposed on a path of a corresponding sensing switch; a plurality of second transistors, being MOSFETs, disposed in respective correspondence with the sensor capacitors, a gate of each of the second transistors being connected to a gate of a corresponding first transistor, and a drain of each of the second transistors being connected to a corresponding integral capacitor; a plurality of fifth transistors, being MOSFETs, disposed in respective correspondence with the sensor capacitors, a gate of each of the fifth transistors being connected to a gate of a corresponding first transistor; a plurality of third transistors disposed on a path of a corresponding fifth transistor, in respective correspondence with the sensor capacitors; and a plurality of fourth transistors disposed in respective correspondence with the sensor capacitors, each of the fourth transistors being connected to a corresponding third transistor to form a current mirror circuit therewith, and a drain of each of the fourth transistors being connected to a corresponding integral capacitor.

According to the present embodiment, the capacitances of the respective sensor capacitors can be detected simultaneously.

According to another embodiment of the present disclosure, provided is an input device. The input device may include: a plurality of sensor capacitors; and a capacitance voltage conversion circuit configured to convert a capacitance of each of a plurality of sensor capacitors into a voltage. The sensor capacitors may be disposed in a matrix type.

Moreover, an arbitrary combination of the above elements or a change of an implementation of the present disclosure in methods and devices are effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an electronic instrument including an input device according to an embodiment.

FIG. 2 is a block diagram illustrating a configuration of a control IC according to the embodiment.

FIG. 3 is a circuit diagram illustrating a detailed configuration example of the control IC.

FIG. 4 is a waveform diagram showing an operation of the control IC according to the embodiment.

FIG. 5 is a circuit diagram illustrating a first modification example of the control IC.

FIG. 6 is a waveform diagram showing an operation of the control IC of FIG. 5.

FIG. 7 is a block diagram illustrating a configuration of the control IC according to a second modification example.

FIG. 8 is a circuit diagram illustrating a detailed configuration example of the control IC of FIG. 7.

FIG. 9 is a waveform diagram showing an operation of a standby mode of the control IC of FIG. 7.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The same or equal elements, members, and processing illustrated in each of the drawings are indicated by the same reference numerals, and a repetitive description is not provided. Moreover, the embodiments do not limit the present disclosure but exemplify the present disclosure, and all features described in aspects or combination thereof may or may not be essential to the present disclosure.

In the specification, ┌a member A being connected to a member B┘ includes the case where the member A and the member B are physically and directly connected to each other, and the case where the member A and the member B are indirectly connected to each other through another member that does not affect an electrical connection.

Likewise, “a member C being prepared between the member A and the member B” includes the case where the member A and the member C are directly connected to each other or the member B and the member C are directly connected to each other, and the case where the member A and the member C are indirectly connected to each other through another member that does not affect an electrical connection or the member B and the member C are indirectly connected to each other through another member that does not affect an electrical connection.

FIG. 1 is a diagram illustrating a configuration of an electronic instrument 1 including an input device 2 according to an embodiment. The electronic instrument 1 includes a Digital Signal Processor (DSP) 6 and a Liquid Crystal Display (LCD) 7, in addition to the input device 2. The input device 2 includes a touch panel 3 and a control IC 4. The touch panel 3 includes a plurality of sensor capacitors C_(S1) to C_(Sn) that are disposed regularly. The sensor capacitors C_(S1) to C_(Sn) are substantially disposed in a matrix type. The control IC 4 is connected to the sensor capacitors C_(S1) to C_(Sn), detects the capacitances of the sensor capacitors C_(S1) to C_(Sn), and outputs data, indicating each of the capacitances, to the DSP 6.

When a user of the electronic instrument 1 touches the touch panel 3 with a finger 5, a pen or the like, the capacitance of the sensor capacitor C_(S) corresponding to touched coordinates is changed. The DSP 6 detects the coordinates touched by the user, on the basis of the capacitances of a plurality of sensor capacitors C_(S). For example, the touch panel 3 may be disposed at a surface of the LCD 7, or disposed at another position of the LCD 7.

The above description has been made on the entire configuration of the electronic instrument 1. Next, the input device 2 will be described in detail.

FIG. 2 is a block diagram illustrating a configuration of the control IC 4 according to the embodiment. The control IC 4 includes a Capacitance/Voltage conversion circuit (C/V conversion circuit) 100, a multiplexer 40, and an Analog/Digital converter (A/D converter) 50, which are integrated on one semiconductor substrate. A portion of the function of the DSP 6 may also be built in the control IC 4.

The C/V conversion circuit 100 converts the capacitance of each of the sensor capacitors C_(S1) to C_(Sn) into a detection voltage corresponding to the capacitance. As described below, the detection voltages of the respective sensor capacitors C_(S) are simultaneously generated and held. A plurality of buffers BUF₁ to BUF_(n) respectively receive detection voltages V_(S1) to V_(Sn), and output the received detection voltages to the multiplexer 40. The multiplexer 40 sequentially selects the detection voltages V_(S1) to V_(Sn) in a time division method. The A/D converter 50 sequentially converts the respective detection voltages V_(S) selected by the multiplexer 40 into digital values D_(OUT).

The C/V conversion circuit 100 converts the capacitances of the sensor capacitors C_(S1) to C_(Sn) into detection voltages V_(S1) to V_(Sn), respectively. The C/V conversion circuit 100 includes a plurality of Capacitance/Current conversion circuits (C/I conversion circuits) 10 ₁ to 10 _(n), a current average circuit 20, and a plurality of Current/Voltage conversion circuits (I/V conversion circuits) 30 ₁ to 30 _(n).

The C/I conversion circuits 10 ₁ to 10 _(n) are disposed in respective correspondence with the sensor capacitors C_(S1) to C_(Sn). The C/I conversion circuit 10 _(i) (where 1≦i≦n) generates a detection current I_(Si) that corresponds to the capacitance of the corresponding sensor capacitor C_(Si), and outputs the detection current I_(Si) to the corresponding I/V conversion circuit 30 _(i), and the current average circuit 20.

The current average circuit 20 averages a plurality of the detection currents I_(S1) to I_(Sn) that are respectively generated by the C/I conversion circuits 10 ₁ to 10 _(n). An averaged detection current I_(AVE) (hereinafter referred to as an average current) is supplied to the I/V conversion circuits 30 ₁ to 30 _(n): I _(AVE)=Σ_(i=1:n) I _(Si) /n  (1)

The I/V conversion circuits 30 ₁ to 30 _(n) are disposed in respective correspondence with the sensor capacitors C_(S1) to C_(Sn). The I/V conversion circuit 30 _(i), converts a difference current I_(DIFFi) (=I_(Si)−I_(AVE)) between the corresponding detection current I_(Si) and the average current I_(AVE) into a voltage, and outputs the converted voltage as the detection voltage V_(Si).

FIG. 3 is a circuit diagram illustrating a detailed configuration example of the control IC 4. In FIG. 3, only portions respectively corresponding to the sensor capacitors C_(S1) and C_(S2) are illustrated.

The C/I conversion circuit 10 _(i) includes a reset switch SW1, a sensing switch SW2, a first transistor M1, and a second transistor M2.

The reset switch SW1 is disposed for initializing the electric charge of a corresponding sensor capacitor C_(Si). For example, the reset switch SW1 may be disposed in parallel to the sensor capacitor C_(Si). When the reset switch SW1 is turned on, the electric charge of the sensor capacitor C_(Si) is discharged and thus initialized. That is, a potential difference between both ends of the sensor capacitor C_(Si) becomes zero. For example, the reset switch SW1 may include an N-channel MOSFET, and when a reset signal RST applied to a gate of the N-channel MOSFET is asserted (high level), the reset switch SW1 is turned on.

The sensing switch SW2 and the first transistor M1 are disposed sequentially and serially between the sensor capacitor C_(Si) and a fixed voltage terminal (herein, a power source terminal). The sensing switch SW2 is a P-channel MOSFET, and when a sensing signal EVALB inputted to a gate of the P-channel MOSFET is asserted (low level), the sensing switch SW2 is turned on.

The first transistor M1 is a P-channel MOSFET. Specifically, a drain of the first transistor M1 is connected to the sensor capacitor C_(Si) through the sensing switch SW2. A source of the first transistor M1 is connected to the power source terminal. In addition, an electric connection is formed between a gate and the drain of the first transistor M1. A charging current I_(CHGi), which corresponds to the capacitance of a corresponding sensor capacitor C_(Si), flows in the first transistor M1.

The second transistor M2 is a P-channel MOSFET of the same type as the first transistor M1, and is connected to the first transistor M1 to form a current mirror circuit therewith. Specifically, a gate of the second transistor M2 is connected to a gate of the first transistor M1, and a source of the second transistor M2 is connected to the power source terminal. A detection current I_(Si), which corresponds to the capacitance of a corresponding sensor capacitor C_(Si), flows in the second transistor M2. When a current ratio (size ratio) of the first and second transistors M1 and M2 is K1, the detection current I_(Si) is expressed as Equation (2): I _(Si) =I _(CHGi) ×K ₁  (2)

The current mirror circuit configured with the first and second transistors M1 and M2 supplies the detection current I_(Si), flowing in the second transistor M2, to a corresponding integral capacitor C_(INTi) in a first direction (charging direction in FIG. 3).

The current average circuit 20 includes a plurality of third transistors M3, a plurality of fourth transistors M4, and a plurality of fifth transistors M5.

The fifth transistors M5 are disposed in respective correspondence with the sensor capacitors C_(Si). Each of the fifth transistors M5 is a MOSFET of the same type as the first transistor M1, and connected to a corresponding first transistor M1 to form a current mirror circuit therewith, thereby generating a current I_(Si)′ which corresponds to a corresponding detection current I_(Si).

The third transistors M3 are disposed on a path of the current I_(Si)′ in respective correspondence to the sensor capacitors C_(Si). Control terminals (gates) of the respective third transistors M3 are connected to each other in common Specifically, a source of each of the third transistors M3 is grounded, and a drain of each of the third transistors M3 is connected to a drain of a corresponding fifth transistor M5.

The fourth transistors M4 are disposed in respective correspondence with the sensor capacitors C_(Si). Each of the fourth transistors M4 is connected to a corresponding third transistor M3 to form a current mirror circuit therewith. The size of each fourth transistor M4 is the same as that of each third transistor M3, in all channels. A current flowing in each fourth transistor M4 is the averaged detection current I_(AVE), and is supplied to a corresponding I/V conversion circuit 30. Specifically, the current average circuits 20 respectively supply a plurality of average currents I_(AVE) to a plurality of integral capacitors C_(INTi) in a second direction (discharging direction in FIG. 3).

Each of the I/V conversion circuits 30 includes an integral capacitor C_(INTi) and an initialization switch SW3 _(i). One end of the integral capacitor C_(INTi) is grounded, and an electric potential thereof is fixed. A corresponding detection current I_(Si) is supplied to the integral capacitor C_(INTi) in the first direction (charging direction), and a corresponding average current I_(AVE) is supplied to the integral capacitor C_(INTi) in the second direction (discharging direction). As a result, the integral capacitor C_(INTi) is charged or discharged by the difference current I_(DIFFi)(=I_(Si)−I_(AVE)).

The initialization switch SW3 _(i) functions as an initialization circuit that initializes the voltage of the integral capacitor C_(INTi) before detection. One end of the initialization switch SW3 _(i) is connected to the integral capacitor C_(INTi) and a reference voltage V_(CM) is applied to the other end of the initialization switch SW3 _(i) by a buffer (voltage follower) 52. The initialization switch SW3 _(i) may be a transfer gate, or may be another switch. When an initialization signal VCM_SW is asserted, the initialization switch SW3 _(i) is turned on. The reference voltage V_(CM), for example, may be a voltage in the vicinity of a central point between a source voltage Vdd and a ground voltage Vss.

In FIG. 3, the multiplexer 40 of FIG. 2 is illustrated as switches SW4 ₁ to SW4 _(n) of respective channels. In addition, in FIG. 3, the A/D converter 50 of FIG. 2 is divided into two A/D converters ADC1 and ADC2. The detection voltages V_(S1), V_(S3), . . . of respective odd channels are allocated to the A/D converter ADC1, and the detection voltages V_(S2), V_(S4), . . . of respective even channels are allocated to the A/D converter ADC2. The outputs of the respective switches SW4 ₁, SW4 ₃, . . . of the odd channels are connected to each other in common, and connected to an input of the A/D converter ADC1. The outputs of the respective switches SW4 ₂, SW4 ₄, . . . of the even channels are connected to each other in common, and connected to an input of the A/D converter ADC2. Further, the detection voltage V_(Si) for all channels may be converted into a digital value by a single A/D converter.

The above description has been made on the detailed configuration of the control IC 4. Next, an operation of the control IC 4 will now be described. FIG. 4 is a waveform diagram showing the operation of the control IC 4 according to the embodiment.

First, the buffer 52 is turned on, and thus, the reference voltage V_(CM) becomes a certain level. In addition, the initialization signal VCM_SW for all channels is asserted, and thus, the plurality of initialization switches SW3 ₁ to SW3 _(n) are turned on (at time t0). Therefore, the voltage levels of the integral capacitors C_(INT1) to C_(INTn) of the respective channels are initialized to the reference voltage V_(CM). When the initialization of the integral capacitor C_(INTi) is completed, the reference voltage V_(CM) becomes 0 V, and the initialization signal VCM_SW is negated, whereupon the initialization switches SW3 ₁ to SW3 _(n) are turned off.

Subsequently, the reset signal RST is asserted, and the plurality of reset switches SW1 ₁ to SW1 _(n) are turned on. Accordingly, the electric charges of the sensor capacitors C_(S1) to C_(Sn) become zero and thus are initialized (at time t1). Next, the reset signal RST is negated, and the reset switches SW1 ₁ to SW1 _(n) are turned off.

Then, the sensing signal EVALB is asserted (low level), and the plurality of sensing switches SW2 ₁ to SW2 _(n) are turned on.

An ith channel is noted. When the sensing switch SW2 _(i) is turned on, the charging current I_(CHGi) flows through the first transistor M1 and the sensing switch SW2 that correspond to the sensor capacitor C_(Si), thereby boosting the electric potential of the sensor capacitor C_(Si). Then, when the electric potential V_(Xi) rises to Vdd−Vth, the first transistor M1 is turned off, and charging is stopped. Vth is a threshold voltage between a gate and a source of the first transistor M1. An amount of electric charge, which is supplied to the sensor capacitor C_(Si) by the charging, is expressed as Equation (3) below and depends on the capacitance of the sensor capacitor C_(Si). Q _(Si) =C _(Si)×(Vdd−Vth)  (3)

That is, the C/I conversion circuit 10 _(i) supplies the current I_(CHGi) to a corresponding sensor capacitor C_(S) until the electric potential of the sensor capacitor C_(Si) reaches a certain level Vdd−Vth.

The C/I conversion circuit 10 _(i), copies the charging current I_(CHGi) to generate the detection current I_(Si) corresponding to a capacitance, and charges the integral capacitor C_(INT) with the detection current I_(Si). Since I_(Si)=K1×I_(CHGi), an amount of electric charge Q_(INTi) supplied to the integral capacitor C_(INTi) is expressed as Equation (4) below: Q _(INTi) Q _(Si) ×K1  (4)

The current average circuit 20 discharges the integral capacitor C_(INTi) due to an average current I_(AVE) of the detection currents I_(S1) to I_(Sn) of the respective channels. An amount of electric charge Q_(INTAVE), which is discharged from the integral capacitor C_(INTi) by the current average circuit 20, is expressed as Equation (5) below: Q _(INTAVE) =Q _(SAVE) ×K1  (5) where Q_(SAVE) is an average value, ΣQ_(Si)/n, of the amounts of electric charges that are respectively supplied to the sensor capacitors C_(S1) to C_(Sn) of the respective channels, and expressed as Equation (6) below: Q _(SAVE) =ΣQ _(Si) /n=ΣC _(Si) /n×(Vdd−Vth)  (6)

When the capacitance of the sensor capacitor C_(Si) is greater than an average value C_(SAVE) of the capacitances of the sensor capacitors C_(S1) to C_(Sn) of the respective channels, since I_(Si)>I_(AVE), the integral capacitor C_(INTi) is charged, and the detection voltage V_(Si) becomes higher by ΔV_(i) than the reference voltage V_(CM) that is an initial value.

$\begin{matrix} \begin{matrix} {{\Delta\; V_{i}} = {\left( {Q_{INTi} - Q_{INTAVE}} \right)/C_{INTi}}} \\ {= {\left( {Q_{Si} - Q_{SAVE}} \right) \times K\;{1/C_{INTi}}}} \\ {= {{\left( {C_{Si} - {\Sigma\;{C_{Si}/n}}} \right)/C_{INTi}} \times K\; 1 \times \left( {{Vdd} - {Vth}} \right)}} \end{matrix} & (7) \end{matrix}$

However, when the capacitance of the sensor capacitor C_(Si) is less than the average value C_(SAVE), namely, when Q_(Si)<Q_(SAVE), since I_(Si)<I_(AVE), the integral capacitor C_(INTi) is discharged, and the detection voltage V_(Si) becomes lower by ΔV_(i) than the reference voltage V_(CM) that is the initial value.

When the capacitance of the sensor capacitor C_(Si) is equal to the average value C_(SAVE), namely, when Q_(Si)=Q_(SAVE), since I_(Si)<I_(AVE), the amount of electric charge of the integral capacitor C_(INTi) is not changed, and ΔV_(i) becomes zero.

The final detection voltage V_(Si) is expressed as Equation (8) below:

$\begin{matrix} \begin{matrix} {V_{Si} = {V_{CM} + {\Delta\; V_{i}}}} \\ {= {V_{CM} + {{\left( {C_{Si} - {\Sigma\;{C_{Si}/n}}} \right)/C_{INTi}} \times K\; 1 \times \left( {{Vdd} - {Vth}} \right)}}} \end{matrix} & (8) \end{matrix}$

In this way, the capacitance changes of the sensor capacitors C_(S1) to C_(Sn) of the respective channels are respectively converted into the detection voltages V_(S1) to V_(Sn), which are respectively held by the integral capacitors C_(INTi) to C_(INTn).

Subsequently, by controlling the switches SW4 ₁ to SW4 _(n) according to an appropriate sequence, the detection voltages V_(S1) to V_(Sn) of the respective channels are respectively converted into digital values by the two A/D converters ADC1 and ADC2.

The above description has been made on the operation of the control IC 4.

According to the C/I conversion circuit 100, each of the capacitances of the sensor capacitors C_(S1) to C_(Sn) may be detected as a voltage proportional to a difference with respect to the average value C_(SAVE) (ΣC_(Si)/n) of the capacitances. Herein, when the number of channels n is sufficiently large and the change of each capacitance is small, the average value C_(SAVE) may be regarded as constant, and thus, the detection voltage V_(Si) is substantially changed linearly according to the capacitance change of the sensor capacitor C_(Si).

The C/I conversion circuit 100 may simultaneously detect the capacitance changes of the respective sensor capacitors C_(S) of the plurality of channels. Accordingly, even when noise affecting the touch panel 3 is being continuously changed with time, common mode noise can be cancelled, thus enhancing the tolerance to noise compared to a conventional method.

Moreover, as clearly seen in Equation (8), the sensitivity of the detection voltage V_(Si) corresponding to the capacitance change may be adjusted with the mirror ratio K1 of the C/I conversion circuit 10 _(i) and current average circuit 20, and the capacitance of the integral capacitor C_(INTi).

The above description has been made on the present disclosure, on the basis of the embodiments. The embodiments are exemplified, and it can be understood by those skilled in the art that various modification examples can be implemented by the combinations of the elements or processors of the embodiments, and are within the technical spirit and scope of the present disclosure. Hereinafter, the modification examples will be described.

First Modification Example

FIG. 5 is a circuit diagram illustrating a configuration of the control IC 4 according to a first modification example. In FIG. 5, only a configuration of a first channel is illustrated. In an actual circuit, a protection diode or a protection pad (not shown) is directly connected to a terminal N_(S) to which a sensor capacitor C_(S) of the control IC 4 is connected, and a parasitic capacitor C_(P) is formed due to the protection diode or the protection pad.

Specifically, a charging current for the sensor capacitor C_(Si) and a charging current for the parasitic capacitor C_(Pi) flow in the first transistor M1. The charging current flowing in the parasitic capacitor C_(Pi) is supplied as expressed in Equation (9) below: Q _(Pi) =C _(Pi)×(Vdd−Vth)  (9) The electric charge is charged into the integral capacitor C_(INTi). When the parasitic capacitances of parasitic capacitors C_(P1) to C_(Pn) of the respective channels are equal, the influences of the parasitic capacitances are cancelled, but when the parasitic capacitances of the respective channels differ, the parasitic capacitances exert a harmful influence on the detection of the sensor capacitors C_(S).

Therefore, the control IC 4 a of FIG. 5 further includes an offset cancel circuit 60 for cancelling the parasitic capacitor C_(Pi) of each channel. The offset cancel circuit 60 supplies a predetermined reference current Iref to a node (line) N_(S) to which a corresponding sensor capacitor C_(S) is connected, for a predetermined duration.

Specifically, the offset cancel circuit 60 includes: a current source 62 that generates the predetermined reference current Iref; a current mirror circuit 64 that copies the reference current Iref and supplies the copied reference current Iref to the node N_(S); and a cancel switch SW5 that is disposed on a path of a output current of the current mirror circuit 64 and is turned on for the predetermined duration. The cancel switch SW5 is turned on for a duration where a control signal PWMB[1] is asserted (low level).

An operation of the C/V conversion circuit 100 a of FIG. 5 will now be described. FIG. 6 is a waveform diagram showing the operation of the control IC 4 a of FIG. 5.

Before the sensing signal EVALB is asserted, in each channel, the control signal PWMB[i] is asserted (low level) for an arbitrary calibration duration T_(i). The calibration durations T_(i), of the respective channels may differ. In the calibration duration T_(i), an electric charge expressed in the Equation below is given to the node N_(S). Q=Iref×T _(i)

When the amount of the electric charge is equal to C_(Pi)×(Vdd−Vth), the influence of the parasitic capacitor C_(Pi) may be cancelled. In this case, the calibration duration T_(i) is provided in the expressed Equation below. T _(i) =C _(Pi)×(Vdd−Vth)/Iref

The calibration duration T_(i) may be common in all channels, and the value of a reference current Iref_(i) may be adjusted for each channel. Alternatively, both the calibration duration T_(i) and the reference current Iref_(i) may be adjusted.

In this way, by disposing the offset cancel circuit 60, the influence of the parasitic capacitor C_(Pi) can be cancelled, and the capacitance change of the sensor capacitor C_(Si) can be detected with high precision.

Second Modification Example

The electronic instrument 1 including the input device 2 is switchable between two states, namely, a standby state (slip state) and a normal state. The input device 2 is set to a standby mode in the standby state, and set to a normal mode in the normal state. In the normal mode, the capacitance change of each sensor capacitor C_(S) of the touch panel 3 is sensed. In the standby mode, in order to reduce the consumption power of the electronic instrument 1, whether a user touches the touch panel 3 is sensed. In the standby mode, when the user touches the touch panel 3, the input device 2 returns to the normal mode. In the present modification example, a description will be made on technology that detects whether the user touches the touch panel 3, at a low consumption power.

As described above, each detection current I_(S) corresponds to the capacitance of a corresponding sensor capacitor C_(S), and the average current I_(AVE) corresponds to an average value of the capacitances of the plurality of sensor capacitors C_(S). When a user touches any one position of the touch panel 3, the average value of the capacitances of the sensor capacitors C_(S) is changed. Such a principle is applied to an input device 2 a according to the second modification example, and the input device 2 a determines whether the user touches at least one of the sensor capacitors C_(S), on the basis of the average current I_(AVE).

FIG. 7 is a block diagram illustrating a configuration of the control IC 4 a according to a second modification example. The control IC 4 a includes a reference capacitor C_(REF), a second C/I conversion circuit 11, a second I/V conversion circuit 31, and a buffer BUF_(n+1), in addition to the elements of the control IC 4 of FIG. 2.

The second C/I conversion circuit 11 is configured similarly to the C/I conversion circuit 10, and generates a reference current I_(REF) corresponding to the capacitance of the reference capacitor C_(REF). The second I/V conversion circuit 31 is configured similarly to the I/V conversion circuit 30, and converts a difference current I_(DIFF)(=I_(REF)−I_(AVE)) between the average current I_(AVE) (which is generated by a current average circuit 20 a) and the reference current I_(REF) into a voltage V_(Sn+1).

The current average circuit 20 a averages the detection currents I_(S) generated by the plurality of capacitance current conversion circuit 10 and the reference current I_(REF), thereby generating the average current I_(AVE)

In the present configuration, since the capacitance of the reference capacitor C_(REF) is fixed, the reference current I_(REF) is constant irrespective of whether the user touches the touch panel 3. Therefore, the difference current “I_(REF)−I_(AVE)” generated by the second I/V conversion circuit 31 has a value corresponding to the average current I_(AVE). Accordingly, in the standby mode, the input device 2 a may determine whether the user touches the touch panel 3, on the basis of the output voltage V_(Sn+1) of the second I/V voltage conversion circuit 31.

In the normal mode, the multiplexer 40 cyclically selects a plurality of voltages V_(S1) to V_(Sn). The A/D converter 50 sequentially converts the voltages V_(S1) to V_(Sn) into respective digital values. In the standby mode, the multiplexer 40 steadily selects the voltage V_(Sn+1) from the second I/V conversion circuit 31. The A/D converter 50 converts only the voltage V_(Sn+1) into a digital value. Thus, in the standby mode, the switching operation of the multiplexer 40 is not required, thus decreasing consumption power.

Moreover, since the A/D converter 50 merely converts the voltage V_(Sn+1) of one channel, consumption power can be considerably reduced compared to a case in which an A/D conversion is performed on all channels. Further, the DSP 6 receiving the output data of the A/D converter 50 detects whether the user touches the touch panel 3 by processing only the voltage V_(Sn+1) of one channel, thus considerably decreasing the number of operations of the DSP6.

In order to further decrease consumption power, in the standby mode, unnecessary circuit blocks such as I/V conversion circuits 30 ₁ to 30 _(n) or buffers BUF₁ to BUF_(n) can be stopped.

FIG. 8 is a circuit diagram illustrating a detailed configuration example of the control IC 4 a of FIG. 7.

The control IC 4 a includes a second reset switch SW6, a second sensing switch SW7, and a second current mirror circuit.

The second reset switch SW6 is disposed in parallel to the reference capacitor C_(REF), and initializes the electric charge of the reference capacitor C_(REF) when turned on. One end of the second sensing switch SW7 is connected to the reference capacitor C_(REF). A sixth transistor M6 and a seventh transistor M7 configure the second current mirror circuit. The sixth transistor M6 is disposed on a path of the second sensing switch SW7, and is connected to the reference capacitor C_(REF) through the second sensing switch SW7. The second current mirror circuit supplies a current I_(REF), which flows in the seventh transistor M7 of an output side of the second current mirror circuit, to a second integral capacitor C_(INTn+1) in the first direction.

The second integral capacitor C_(INTn+1) is disposed in correspondence with the reference capacitor C_(REF), and the electric potential of one end of the second integral capacitor C_(INTn+1) is fixed. An initialization switch SW3 _(n+1), which is a second initialization circuit, is disposed for initializing the voltage of the second integral capacitor C_(INTn+1).

The current average circuit 20 a averages a plurality of detection currents I_(Si)′ to I_(Sn)′ (which flow in the plurality of sensor capacitors C_(S1) to C_(Sn), respectively) and a reference current I_(REF)′ flowing in the reference capacitor C_(REF), thereby generating the average current I_(AVE). The current average circuit 20 a supplies the average current I_(AVE) to the plurality of integral capacitors C_(INT1) to C_(INTn) and the second integral capacitor C_(INTn+1) in the second direction.

The current average circuit 20 a includes an eighth transistor M8, a ninth transistor M9, and a tenth transistor M10, in addition to the elements of the current average circuit 20 of FIG. 3. A gate of the tenth transistor M10 is connected to a gate of the sixth transistor M6. The eighth transistor M8 is disposed on a path of the tenth transistor M10.

The ninth transistor M9 is connected to the eighth transistor M8 to form a current mirror circuit therewith, and a drain of the ninth transistor M9 is connected to the second integral capacitor C_(INTn+1).

In the standby mode, whether a user touches the touch panel 3 is determined on the basis of the voltage V_(Sn+1) of the second integral capacitor C_(INTn+1).

The reference capacitor C_(REF) is built in the control IC 4 a, and may be preferably configured as a variable capacitor. As described below, the voltage V_(Sn+1) has a level corresponding to a difference between the capacitance of the reference capacitor C_(REF) and an average capacitance of the capacitances of the sensor capacitors C_(S). Therefore, by varying the capacitance of the reference capacitor C_(REF), the range of the voltage V_(Sn+1) can be matched with a circuit of a next stage.

The above description has been made on the configuration of the control IC 4 a. Next, an operation of the control IC 4 a will be described. FIG. 9 is a waveform diagram showing an operation in the standby mode of the control IC 4 a of FIG. 7.

In the standby mode, an operation before time t2 is as shown in the waveform diagram of FIG. 4.

First, the buffer 52 is turned on, and thus, the reference voltage V_(CM) becomes a certain level. In addition, the initialization signal VCM_SW for all channels is asserted, and thus, the plurality of initialization switch SW3 ₁ to SW3 _(n+1) is turned on (at time t0). Therefore, the voltage level of the integral capacitors C_(INT1) to C_(INTn+1) of the respective channels is initialized to the reference voltage V_(CM). When the initialization of the integral capacitor C_(INT) is completed, the reference voltage V_(CM) becomes 0 V, and the initialization signal VCM_SW is negated, whereupon the initialization switches SW3 ₁ to SW3 _(n) are turned off.

Subsequently, the reset signal RST is asserted, and the plurality of reset switches SW1 ₁ to SW1 _(n) and SW6 are turned on. Accordingly, the electric charges of the sensor capacitors C_(S1) to C_(Sn) become zero and thus are initialized (at time t1). Next, the reset signal RST is negated, and the reset switches SW1 ₁ to SW1 _(n) and SW6 are turned off.

Then, the sensing signal EVALB is asserted (low level), and the plurality of sensing switches SW2 ₁ to SW2 _(n) and SW7 are turned on. As a result, the C/I conversion circuit 10 _(i) supplies the current I_(CHGi) to the corresponding sensor capacitor C_(S), until the electric potential of the sensor capacitor C_(Si) reaches a certain level, Vdd−Vth, and the C/I conversion circuit 11 supplies a current I_(CHGn+1) to the corresponding reference capacitor C_(REF) until the electric potential of the reference capacitor C_(REF) reaches the certain level, Vdd−Vth.

An n+1st channel is noted. The current I_(REF), which is supplied to the second integral capacitor C_(INTn+1) in the first direction, has a constant level regardless of whether the user touches the touch panel 3. The current I_(AVE), which is supplied to the second integral capacitor C_(INTn+1) in the second direction, is changed according to whether the user touches the touch panel 3.

When the capacitance of the reference capacitor C_(REF) is equal to an average capacitance of the capacitances of the plurality of sensor capacitors C_(S), the charging current I_(REF) of the second integral capacitor C_(INTn+1) becomes equal to the discharging current I_(AVE) of the second integral capacitor C_(INTn+1), and thus, the electric potential V_(Sn+1) of the second integral capacitor C_(INTn+1) becomes equal to the reference voltage V_(CM). In addition, when the capacitance of the reference capacitor C_(REF) is greater than the average capacitance of the capacitances of the plurality of sensor capacitors C_(S), since I_(REF)>I_(AVE), the electric potential V_(Sn+1) of the second integral capacitor C_(INTn+1) becomes equal to the reference voltage V_(CM).

However, when the capacitance of the reference capacitor C_(REF) is less than the average capacitance of the capacitances of the plurality of sensor capacitors C_(S), since I_(REF)<I_(AVE), the electric potential V_(Sn+1) of the reference capacitor C_(REF) becomes lower than the reference voltage V_(CM).

Herein, when the user touches the touch panel 3, the capacitance of at least one of the sensor capacitors C_(S) increases, and thus, the average capacitance of the capacitances of the sensor capacitors C_(S) increases. Accordingly, the electric potential V_(Sn+1) of the reference capacitor C_(REF) has a level that varies according to whether the user touches the touch panel 3.

After the sensing signal EVALB is asserted, a switch AD_SWn+1 of a multiplexer 40 a is turned on, and thus, the voltage V_(Sn+1) is inputted to the A/D converter ADC2 and converted into a digital value. The DSP 6 determines whether the user touches the touch panel 3 on the basis of the digital value, and when detecting the touch, the DSP 6 performs the normal mode.

As shown in FIG. 9, in the standby mode, a plurality of switches AD_SW1 to AD_SWn are always turned off, and thus, the multiplexer 40 a steadily selects the voltage V_(Sn+1). Accordingly, the switching of the switch is not required. Further, an operation of each of the buffers BUF₁ to BUF_(n) is not required. Additionally, an A/D conversion may be performed only once in an n+1st channel. Moreover, data is transferred from the control IC 4 to the DSP 6 for one channel. Accordingly, the consumption current of the control IC 4 can be greatly reduced.

Further, the DSP 6 determines whether the user touches the touch panel 3 with data for one channel. The determination merely compares digital data, corresponding to the voltage V_(Sn+1) from the DSP 6, with a threshold voltage, and thus is a very simple operation. Accordingly, the number of operations of the DSP 6 can be considerably reduced.

Other Modification Examples

In the above embodiments, the touch panel 3 where the plurality of sensor capacitors C_(S) are substantially disposed in the matrix type has been described as an example, but the application of the C/V conversion circuit 100 is not limited thereto. For example, the C/V conversion circuit 100 may be applied even to an X-Y type of touch panel, in which case the capacitances of a plurality of row sensor electrodes and the capacitances of a plurality of column sensor electrodes may be detected simultaneously.

The C/V conversion circuit 100, which has been described in the above embodiments, may be switched in its upper and lower positions. In this case, it can be understood by those skilled in the art that the P-channel MOSFETs and the N-channel MOSFETs may be appropriately switched in their disposed positions. Herein, charging and discharging are changed therebetween, but their fundamental operations are not changed. Furthermore, some transistors may be respectively replaced by bipolar transistors instead of MOSFETs.

In the above embodiments, the capacitance voltage conversion circuit 100 has been described as being applied to the input device using the capacitance change, but the use of the capacitance voltage conversion circuit 100 is not limited thereto. For example, the capacitance voltage conversion circuit 100 may be applied to a microphone such as a capacitor type microphone, in which a capacitor is formed by a diaphragm electrode and a back plate electrode and the capacitance of the capacitor is changed by a sound pressure.

Moreover, since the capacitance voltage conversion circuit 100 amplifies and detects a very small capacitance change, the capacitance voltage conversion circuit 100 may be applied to various applications.

In the above embodiments, the capacitance voltage conversion circuit 100 has been described as being integrated on one semiconductor integrated circuit, but is not limited thereto. As another example, each circuit block may be configured with a chip component or a discrete element. The number of blocks to be integrated may be determined according to an applied semiconductor manufacture process, cost, characteristic, etc.

The input device according to the above embodiments may be applied to various electronic instruments, which include various input devices, such as personal computers, PDAs, digital cameras, remote controllers of CD players, etc., in addition to portable terminals that have been described in the above embodiments.

According to the embodiments of the present disclosure, the capacitance voltage conversion circuit can simultaneously detect the capacitances of the plurality of sensor electrodes.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the novel methods and apparatuses described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures. 

What is claimed is:
 1. A capacitance voltage conversion circuit which converts a capacitance of each of a plurality of sensor capacitors into a voltage, the capacitance voltage conversion circuit comprising: a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each of the capacitance current conversion circuit configured to generate a detection current corresponding to a capacitance of a corresponding sensor capacitor; a current average circuit configured to average detection currents, which are generated by the capacitance current conversion circuits, to generate an average current; a plurality of current voltage conversion circuits disposed in respective correspondence with the sensor capacitors, each of the current voltage conversion circuit configured to convert a difference current between a corresponding detection current and the average current into a first output voltage, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of a difference current between the reference current and the average current; a reference capacitor; and a second capacitance current conversion circuit configured to generate a reference current corresponding to a capacitance of the reference capacitor.
 2. The capacitance voltage conversion circuit of claim 1, further comprising: a second current voltage conversion circuit configured to convert the difference current between the reference current and the average current into a second output voltage.
 3. The capacitance voltage conversion circuit of claim 2, further comprising: a multiplexer configured to receive respective first output voltages of the plurality of current voltage conversion circuits and the second output voltage of the second current voltage conversion circuit, and to select one of the received first and second output voltages; and an Analog/Digital converter configured to convert the one of the received first and second output voltages, selected by the multiplexer, into a digital value, wherein in the standby mode, the multiplexer selects the second output voltage of the second current voltage conversion circuit.
 4. The capacitance voltage conversion circuit of claim 3, wherein in the standby mode, the plurality of current voltage conversion circuits are stopped.
 5. The capacitance voltage conversion circuit of claim 1, wherein the reference capacitor is a variable capacitor.
 6. A capacitance voltage conversion circuit which converts a capacitance of each of a plurality of sensor capacitors into a voltage, the capacitance voltage conversion circuit comprising: a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each of the capacitance current conversion circuit configured to generate a detection current corresponding to a capacitance of a corresponding sensor capacitor; a current average circuit configured to average detection currents, which are generated by the capacitance current conversion circuits, to generate an average current; a plurality of current voltage conversion circuits disposed in respective correspondence with the sensor capacitors, each of the current voltage conversion circuit configured to convert a difference current between a corresponding detection current and the average current into a first output voltage, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of the average current; a reference capacitor; a second capacitance current conversion circuit configured to generate a reference current corresponding to a capacitance of the reference capacitor; and a second current voltage conversion circuit configured to convert a difference current between the reference current and the average current into a second output voltage, wherein the current average circuit averages the reference current and the detection currents which are respectively generated by the plurality of capacitance current conversion circuits, thereby generating the average current.
 7. The capacitance voltage conversion circuit of claim 6, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of an output voltage of the second current voltage conversion circuit.
 8. The capacitance voltage conversion circuit of claim 7, further comprising: a multiplexer configured to receive the respective first output voltages of the plurality of current voltage conversion circuits and the second output voltage of the second current voltage conversion circuit, and to select one of the received first and second output voltages; and an Analog/Digital converter configured to convert the one of the received first and second output voltages, selected by the multiplexer, into a digital value, wherein in the standby mode, the multiplexer selects the second output voltage of the second current voltage conversion circuit.
 9. The capacitance voltage conversion circuit of claim 8, wherein in the standby mode, the plurality of current voltage conversion circuits are stopped.
 10. The capacitance voltage conversion circuit of claim 6, wherein the reference capacitor is a variable capacitor. 